1. Field of the Invention
The present invention relates generally to a tensegrity structure. An embodiment of the invention relates to a tensegrity unit that has no loose bars or cables in a collapsed state, and that may be easily and rapidly deployed. Several tensegrity units may be coupled together to assemble a tensegrity structure.
2. Description of Related Art
A tensegrity unit is a self-stressed equilibrium network in which a continuum of tension members (e.g., cables) interacts with a discontinuous system of compression members (e.g., bars) to provide the unit with structural integrity. The tension members may be cables, lines, chains, or other similar devices that sustain tension forces. A continuum of tension members means that tension members may directly interact with, or be coupled to, other tension members. A discontinuous system of compression members means that compression members may not directly interact with, or be coupled to, other compression members. The compression members may be rigid members such as bars, poles, rods, or other similar devices that are capable of sustaining compressive forces. Tensile forces rather than compressive forces may primarily provide structural integrity (i.e., shape, strength, etc.) in a tensegrity structure,
Compression members and tension members may form a tensegrity unit. A simple tensegrity unit may be a polyhedron formed of two base polygons held apart from one another. The tensegrity unit may have a skewed prism shape when deployed. For example, 2n tension members may define the edges of a first and second base polygon, where n is the number of vertices in each polygon. Each base polygon may be a basic geometric shape such as a triangle, square, rectangle, trapezoid, etc. The first and second polygons may be similar in size, or the first and second polygons may have different sizes. The first and second base polygons may be spaced apart by n compression members. Compression members may be positioned along diagonals joining vertices of the first base polygon to the second base polygon. An additional n tension members may join corresponding vertices of the first and second base polygons. In such an arrangement, 3n tension members are needed to form a tensegrity unit.
FIG. 1a depicts one type of tensegrity unit. Tensegrity unit 10 has three compression members 12 and nine tension members. The tension members include three separate upper tension members 14, three separate lower tension members 16, and three separate side tension members 18. Tensegrity unit 11 of FIG. 1b has four compression members 13 and twelve tension members 15. Thus, tensegrity units 10 and 11 both have n compression members and 3n tension members.
Tensegrity units may be joined to form tensegrity systems or structures. A tensegrity system may form a three-dimensional structure such as a dome, tower, etc. The geometry of a tensegrity system may depend on the geometric configuration of the individual tensegrity units of the system and the way the units are coupled together.
Tensegrity structures may be collapsible and/or deployable. Releasing tension in a tensegrity unit may allow compression members to collapse into substantially the same plane. In such a configuration, a tensegrity unit may have no rigid structure. When collapsed, the total size of a tensegrity unit may be minimized. Tension may be applied to tension members of a tensegrity unit to deploy the tensegrity unit.
A tensegrity structure may be created so that removing tension from at least some tension members allows the entire structure to be folded for storage, transport, etc. A collapsed tensegrity structure could be deployed by applying tension to appropriate tension members. In practice, a collapsible/deployable tensegrity structure formed by joining tensegrity units in 2 directions has been elusive. For structures of significant size, issues such as tangling of tension members and collisions between compression members may become quite difficult to solve.
A number of tensegrity units may be joined together to form a tensegrity structure or tensegrity network. Individual tensegrity units may be coupled together in at least two different ways. Tensegrity units may be interlaced so that compression member independence is maintained throughout the structure. One way to maintain compression member independence is to couple an end of a compression member of a first unit to a tension member of a second unit. Joints of this type of union may be relatively simple, and worn parts may be easily replaceable. Maintenance may also be reduced due to wear on joints being minimized. It is believed that wear may be minimized on joints of this configuration due to the lack of friction between compression members. Methods for forming a tensegrity structure are disclosed in U.S. Pat. Nos. 3,354,591 and 3,063,521 to Fuller, both of which are incorporated by reference as if fully set forth herein.
Tensegrity structures with no compression member-to-compression member connections may be geometrically deformable. Such structures may be relatively insensitive to inaccuracies in tension member lengths and/or compression member lengths. In some other types of deployable structures, such as scissor structures, minor inaccuracies in tension member lengths and/or compression member lengths may significantly affect structure assembly and load bearing ability.
A second way to assemble tensegrity units together involves attaching tensegrity units together at nodes (or vertices) so that there is a compression member-to-compression member connection. This type of connection only partially conforms to the definition of tensegrity since compression member independence is lost. Such connections may require complex joints if the resulting structure is intended to be collapsible. Rigid connection of compression members may inhibit structure collapse. U.S. Pat. No. 5,642,590 to Skelton, which is incorporated by reference as if fully set forth herein, discloses a tensegrity structure utilizing compression member-to-compression member connections.
Tensegrity structures may possess a high level of structural redundancy. The structural redundancy may inhibit collapse of the tensegrity structure if one or several units should fail. The tensegrity structure may retain a large percentage of load bearing capacity even if one or more members fail.
Two methods have been used to deploy tensegrity units. Deploying a tensegrity unit may include expanding the unit and establishing static equilibrium between the members of the unit such that the unit remains in the expanded state. For example, tensegrity unit 10 in FIG. 1a may be deployed by decreasing the length of one or more of tension members 14, 16, 18 or by increasing the length of one or more of compression members 12. In each of these methods, other steps may also be required, such as positioning the members before applying tension. Likewise, collapsing tensegrity unit 10 may be accomplished by releasing tension in the unit. Methods to collapse tensegrity unit 10 may include increasing the length of one or more tension members 14, 16, 18, or decreasing the length of one or more compression members 12.
Experiments were conducted to determine differences between deploying/collapsing tensegrity units having n compression members and 3n tension members by adjusting tension members and by adjusting compression members. Small-scale triangular based and square based tensegrity units were examined. For one set of experiments, the lengths of compression members (bars) were kept constant while the lengths of tension members (cables) could be modified. The results indicated that releasing the cables in different sequences resulted in different modes of collapsing the unit. In general, by releasing the cables on the top base of the unit first, the unit tends to collapse on its support plane (e.g., the ground). As the unit collapses, the bars may take a turn around an axis of symmetry of the original unit. Packaging a unit in a compact configuration may require that all bars be carefully aligned after collapse to avoid entanglement with loose cables. Aligning bars in this manner may be difficult to achieve with the unit on the ground, especially since the bars tend to collapse in a symmetric pattern around the center of the unit.
When side cables are released first, tension from cables push the collapsing unit to an upright position. With careful handling, bars may be held together so that, when all the cables that allow for the complete collapse of the unit are released, the bars remain parallel to each other. The unit may then be packaged in a bundle. Care must be taken with loose ends of released tension members to avoid tangling of the tension members. Positioning the compression members and the tension members and setting the final tension member may be tedious and difficult when the unit is redeployed. Applying tension may require special tools.
Experiments were conducted using scale models of two curved tensegrity structures composed of three and nine units, respectively. The units had triangular bases. These structures were allowed to collapse by systematically releasing cables. Results indicated that the collapse of the entire structure by releasing only side cables was not possible because of the synergetic action of the units. To collapse the structure, it was necessary to release both side and top cables. The structure collapsed on its support plane in a symmetric but complex configuration. Packaging the collapsed structure in a bundle from this configuration was cumbersome. Deploying the collapsed structure to its initial geometry was even more difficult due to frequent cable entanglement. In addition, it is believed that for a full-scale structure of either of these configurations, collision of bars and the total weight of the structure may pose significant burdens to the application of the method.
Triangular base units with telescoping bars were used to examine collapse behavior of tensegrity units by adjusting lengths of compression members. The lengths of tension member cables were kept constant. The cables were permanently attached to the bars. When the bar length was reduced so that it became the same length or slightly shorter than the side cable length, the unit lost its rigidity but did not entirely collapse on the ground. During the process, if the bars were held together, the unit could collapse to an upright or vertical position. In the collapsed configuration, side cables could be kept almost straight, possibly reducing the likelihood of entanglement. Shortening or elongation of the telescopic bars, the length of which were manually controlled, however, required locking and unlocking of each bar. In addition, when deploying the unit, locking the last bar in place to establish appropriate tension in the unit was difficult with considerable tension already present within the unit.